Materials which closely mimic the ultrasonic propagation characteristics of human tissue are employed in imaging phantoms and other test objects for use with ultrasound scanners. These phantoms may be used to carry out performance checks on ultrasound scanners. Phantoms may also be used for training or testing student technologists in the operation of ultrasound scanners or the interpretation of ultrasound images produced by such scanners.
Ideally, such material should be capable of mimicking soft human tissue with respect to at least three characteristics: speed of sound, ultrasonic attenuation, and ultrasonic backscattering. Additionally, the attenuation coefficient should be approximately proportional to the ultrasonic frequency. In other words, the variation of the attenuation coefficient with respect to frequency, or the attenuation coefficient slope, should remain constant for varying ultrasonic frequencies. The attenuation coefficient should extrapolate to nearly zero as the frequency reduces to zero. These characteristics of human tissue should be maintained at all frequencies in the typical range of ultrasonic scanners, from 1-10 MHz. Moreover, the variation of these characteristics within the range of room temperature should be small. Additionally, these materials should be stable in time and invulnerable to reasonable environmental fluctuations. They should also be free of any pockets of air or gas. Furthermore, the bulk properties of the material should be the same throughout the volume of a particular phantom or phantom section.
A tissue mimicking material satisfying the above characteristics was disclosed in U.S. Pat. No. 4,277,367, to Madsen, et al., entitled Phantom Material and Methods in which both the speed of sound and the ultrasonic attenuation properties could be simultaneously controlled in a mimicking material based on water based gels, such as those derived from animal hides. In one embodiment, ultrasound phantoms embodying the desired features for mimicking soft tissue were prepared from a mixture of gelatin, water, n-propanol and graphite powder, with a preservative. In another embodiment, an oil and gelatin mixture formed the basis of the tissue mimicking material.
Tissue mimicking material is typically used to form the body of an ultrasound scanner phantom. This is accomplished by enclosing the material in a container which is closed by an ultrasound transmitting window cover. The tissue mimicking material is admitted to the container in such a way as to exclude air bubbles from forming in the container. In addition to the tissue mimicking material itself, scattering particles, spaced sufficiently close to each other that an ultrasound scanner is incapable of resolving individual scattering particles, and testing spheres or other targets may be located within the phantom container, suspended in the tissue mimicking material body. Such an ultrasound phantom is useful in evaluating the ability of ultrasound medical diagnostic scanners to resolve target objects of selected sizes located throughout the tissue mimicking material. The objective is for the ultrasound scanner to resolve the testing spheres or other targets from the background material and scattering particles. This type of ultrasound phantom is described in U.S. Pat. No. 4,843,866, to Madsen, et al., entitled Ultrasound Phantom.
U.S. Pat. No. 5,625,137 to Madsen, et al. discloses a tissue mimicking material with intrinsic very low acoustic backscatter coefficient that may be in liquid or solid form. A component in both the liquid and solid forms is a filtered aqueous mixture of large organic water soluble molecules and an emulsion of fatty acid esters, which may be based on a combination of milk and water. Hydroxy compounds, such as n-propanol, can be used to control the ultrasonic speed of propagation through the material and a preservative from bacterial invasion can also be included. The use of scattering particles allows a very broad range of relative backscatter levels to be achieved.
Ultrasound scanners for medical imaging are available from several manufacturers, and in various models, with corresponding variations in the performance of the scanners. There are several performance characteristics of ultrasound scanners that affect the ability of the scanner to provide useful information to medical personnel. One characteristic is the ability of the scanner to delineate the intersection of the plane of symmetry of the scan slice with the three-dimensional boundary of an object regardless of its shape, which is important in the ability of the scanner to provide morphological information concerning diagnostic targets such as tumors. The scanner should also be able to provide contrast resolution of large objects, such as large tumors, which have relatively low ultrasound contrast with respect to surrounding tissue. The lateral and axial spatial resolution capability of the scanner determines the ability of the scanner to provide a defined image of body structures in the lateral and axial dimensions of the scan slice. Other performance characteristics of scanners include the maximum depth into tissue of the scan slice at which visualization of structures in the patient can be obtained, the image gray level uniformity, and the accuracy of distance measurements between reflectors in the tissue that may be obtained from the scan slice that is displayed by the scanner. A particular scanner model may have superior performance in one of these characteristics compared to other scanners and lesser performance with respect to others. To some extent these performance criteria are not precisely quantifiable, but relate to the ability of human observers to obtain useful information from the displayed images. In addition, a particular scanner may change in its performance over time, due to loss of calibration or degradation of components, and such changes may affect some of the performance criteria more than others. It would thus be useful to have a standardized test of scanners which would allow rapid and convenient testing of all of these performance criteria of the scanner and reliable comparative testing of various models of scanners, and the ability to quantify the performance of scanners over time to detect any changes in performance.